extract site_name and site_description from theme settings into standalone settings

site_name and site_description are shop identity, not theme concerns.
They now live in the Settings table as first-class settings with their
own assigns (@site_name, @site_description) piped through hooks and
plugs. The setup wizard writes site_name on account creation, and the
theme editor reads/writes via Settings.put_setting. Removed the
"configure your shop" checklist item since currency/country aren't
built yet. Also adds shop name field to setup wizard step 1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/admin_layout_hook.ex

@@ -30,6 +30,8 @@ defmodule BerrypodWeb.AdminLayoutHook do
       |> assign(:site_live, Settings.site_live?())
       |> assign(:email_configured, Berrypod.Mailer.email_configured?())
       |> assign(:theme_settings, theme_settings)
+      |> assign(:site_name, Settings.site_name())
+      |> assign(:site_description, Settings.site_description())
       |> assign(:generated_css, generated_css)
       |> assign(:attention_count, ActivityLog.count_needing_attention())
       |> Phoenix.LiveView.attach_hook(:set_current_path, :handle_params, fn _params,